Frequency-modulated carrier signal receiver



Feb. 6, 1951 E. H. ARMSTRONG FREQUENCY-MODULATED CARRIER SIGNAL RECEIVER 2 Sheets-Sheet 1 Filed Jan. l2, 1940 S QJ lf d N RWMQY. mm m Nrw/R WM m WW. A my B Feb. 6, 1951 E. H. ARMSTRONG EREQuENcY-MonULATED CARRIER SIGNAL RECEIVER Filed Jan. 12, 1940 2 Sheets-Sheet 2 REQ All "II" SEK Patented Feb. 6, 1951 FREQUENCY-MODULATED CARRIER SIGNAL RECEIVER Edwin H. Armstrong, New York, N. Y.

Application January 12, 1940, Serial No. 313,499 s olaiins. (cl. .25o-2o) This invention relates to improvements in receivers for frequency modulated waves in radio signaling, particularly for use in the reception of the Wide band type of transmission described in my U. S. Patent #1,941,069.

With this system of signaling the most troublesome type vof interference is that caused by the ignition system of motor vehicles, and in my U. S.' Patents No. 2,116,501 and No. 2,116,502 there are described methods of reception which are particularly adapted to the suppression of this type of interference.

It is the purpose of this specification to describe improvements in these methods which,i result in substantial betterment in the signal to noise ratio that can be obtained under conditions of severe interference for ignition systems or, in fact, of anyV impulse type Vof disturbance.

Fig. I-illustrates the general principle vof the invention, and Figs. II, III and IV illustrate more detailed arrangements of the means employed,

corresponding circuit elements being indicated by the same reference numerals.

The invention will be `understood from the following explanation. Inrmy U. S. Patent No.

Y2,116,502 there is described a method of reennumber of ways, but perhaps the simplest method, as explained in my U. S. Patent No. 2,116,502, is

' the use of a tuned circuit adjusted to be resonant to the midfrequency of the band and suiiicientlydamped by resistance to pass in some degree the' desired band. It is, however, not possible with this simple arrangement to carry the reenforcing process out beyond a rcertain point, as transient oscillations set up in the tuned circuit as the frequency of the incoming signal is varied Widely results in distortion.

It is the purpose of the present invention to remedy this difficulty by automatically varying the. damping of the selective means used for the reenforcing process, so .that when the frequency fiiilieifeeaiites wie entre. reduite@ i1i- (excitation are much reduced. The reenforcement of the midfrequencies may be accomplished in a ran audio amplier.

damping of the selective means will be increased. It isa further object of the invention to cause the damping of the circuit .to be increased with the rate of change of the frequencyof the re-`v ceived signal, since the distortion produced whenk the selective circuitused in the reenforcing process is too lightly damped depends not only A,on

the extent of the change of frequency but also on its rate.

The operation of the invention may best be understood by reference to the figures which form a part of this specification. Referring now to Fig. I, the antenna is represented by I, 2, connected to a rectifier or converter 3. 4 represents an oscillator for heterodyning the frequency of the received current down to some intermediate tuned to the midfrequency ofthis band. 8 represents a damping resistance, 9 an amplifier for the intermediate frequency, and I0 a vacuum" tube whose plate circuit is connected in series with the resistance 8 across the tuned circuit 6,` 1. Further reference to the function of this' tube Willbe made later. The output of the ain-v plifier VEl is supplied to another amplifier I I Whose" output in turn is connected to a current limiter I2. The remainder of the receiver I3, I4, I5, I6 is 'arranged in the usual manner for frequency modulation reception, I3r representing an ampli# lier, I4 a device for converting'frequency changes into amplitude changes, I5 a detector, and I6 tiiied audiocurrent flows.` 2|, 22 and 23 represent a filter for removing the audio currents from the output of the rectifier leaving only the direct current component. 24 represents a source of bias voltage for the grid of the vacuum tube I0.' -Referring now to the vacuum tube I0, it will be observed vthat the resistance 8 and the plate resistance of this tube are connected in series across the tuned .circuit 6, 1. The resistance of the tube I0 depends among other things upon the :grid `potential and hence by varying this voltage itis possible to control the damping of the-circuit 6, 1.

In accordance with this invention, the damping ofthe circuit is controlled ina manner depending with@ .extent or thararletlan ofthe receivedfree;

Connected to the output oi-V ythe-audio amplifier or to'any convenient point of *the amplier is a transformer I1 whose secondaryY `is rconnected to a pair of rectiflers I9, 20. represents a resistance through which the rec-- quency by rectifying the audio frequency currents resulting from the deviations of frequency and applying the direct current component of the voltage to the control of the grid potential of the tube l0. It will be observed from the diagram that the greater the amplitude of the audio current the greater the rectified voltage, and hence the more positive the grid potential of the tube l becomes. This results in -a yloxvergplate 4 resistance more :effectually damping lthe circuit 6, 1 thus causing a corresponding increase in the width of the band of frequencies passed thereby. By proper choice of the resistance -8,.the..char ac teristics of the tube IU andits biasingvoltagell! and the level of the intermediate frequency voltage developed across the circuitl 6,"any `desired characteristic of the damping withrespect to the variation in frequency of the'received signal may be obtained. As the level of the intermediate frequency voltage is of 'some importance, it will usually Ybe founddesirableto make use of an-automatic amplification control.

.Fig. .II shows .a modification .of the .arrangement .for damping vthe 'tuned circuit. .In .this gure asimple .diodej 2.5. replaces the tube ID, .and therectied voltage'from the outputof I9, 2n is applieddirectlyin,series vvvththecathode current of gthe. diode, .a ,battery r,26 being Vincluded in the diode circuit. lThe amount of the rectiedcontrelvoltageappliedto the diode maybeadiusted bymeans oftheadiustable contactfZL'as shown. n.additionaldamping.resistance 28 is included, but .in all other respects ;the operation Iof the system is the same asfthatof Fig. -I.

-..Fig. LIII 4illustrates .a 4modification .of zthe arrangement of `Fig. II in .which fthe grid of thel amplifying H,tube .-,--connected #to the -tuned circuit isusedaas'the-meansof damping the circuit. f' This is accomplished --asfshown in the ydiagram vby applying the control "voltage -Jacross v=a 'condenser 2.9and causing therectiedaudio voltage to make the grid'ofthe amplifyingr 'tube Brpositive,-'thereby drawing grid vcurrent -and :damping :fzthe circuit. To-prevent excessive current through the tube A9 azdroppingresistance 30.shunted by 2a condenser 3l may be 4included in the rscreencircuit. `In all other respects the-operationis similar to vthat of Figure II.

.-As vhas already been stated, :the vdisturbance caused by the freefoscillation set up in the tuned circuit-depends notonly on 'thefextent of thecleviation of 4frequency but also :on vits'frate. It vhas been found advantageousrto make the additional dampingintroduced in the 'circuit dependent not only on v.the `extent fof -the ,-deviation, 'but also on its rate -of variation. This' can rbie :readily accom# pjlishedrby .the earrangemefntfindicated 'in Fig. IV, which illustrates 'the rectifying part rof-:tl're 'damp VV damping v"eiect 'can be made substantially pro# portionalrto the :frequency'as Well-aste the ampli-f tude' at .which the i transmitted -wave 'is caused to swing. This results v'in "a substantial fimprove` mentiintthe operation o'ftlre system. 'In the ex-1 ampleiillustratedia resistor 233 is connected across the primary of "transformer "l1,

"In order vto minimize lthe number o'f adjust-A ments'it is'desirable to-inc'lude'an automatic volume control of the amount of intermediate fre' however, to be conned to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. A radio receiving system for frequency modulated signal currents comprising, in combination, a resonant circuit adapted to pass a band of frequency modulated currents, damping means connected across theterminals Aof v1said :resonant circuit, a ffrequency .modulationfdetector ffor demodulating the frequency modulated currents,

and means responsive to the demodulated currents forautomatically controlling the damping .effect-of said clamping means in accordance with the extent of the frequency variations of the :received signal lwhile maintaining the resonant .frequency of said .circuit substantially constant.

'-2. rA radio'receiving system for frequency modulatedgsignal currents comprising, in combination, a resonant circuit adapted to pass a band of frequency vmodulated. Icurrents,.,a vacuum Ytubeha-vingits ,plateconnected `to one `.terminal of said circuit .and its cathode to `the .other terminal thereof,.and -meansfor automatically decreasing Y the plate resistance :ofifsaid ,tube in accordanceA with an-increase inthe range of frequency'variationscf ,the Yreceivedsignal and .in accordancev with .the rate of increase of such range.

:3. ,A radio receiving systemfor frequency modtb lated signal` currents comprising,fin combination, afresonant circuit vadaptedto passa bandfof'frequency modulated Currentsdamping meansgcon nected across the terminals of :said 4resonant circuitgand means .for automaticallycontrollingthe damping eiectofsaid Vdamping means in accord' ance with the irate-ofichange ofthe frequency variations vof ,the ,received zsignal and `also Withv the 'extentcoi said frequency variations.

il. Ara-dio .receiving systemasset'forth in claim 3 -inwhich'the meansfor controlling the damping .eect comprises '.'a lter' ievicefwhosei-react ance decreases .as .the i frequency increases.

5. A frequency-modulated carrier-signal re'- ceiver Acomprising, 4'a :carrier-'frequency 'signaltranslating channel -including a .band-pass -'se lector, means'for adjusting said Vband-pass `se lector to control `the Width of the pass ban'd thereof, a 'frequency-modulation detector coupled to'said channel, means responsive to the detected signal amplitude for deriving a control effect, and means for utilizing'said'control veect to control said adjusting means'to adjust'the pass band of said selector ldirectly in accordance with the degree of'modulation of a received carrier signal. f6.ln a frequency modulation receiver of the 'superheterodyne type including a signal collector,j a Ic'onverter,V at vleast vone -intermediate frequency amplier, an amplitude -modulation limiter,- means for 'deriving Vmodulation voltage from Vth'e amplified frequency-modulated, intermediate fre-V quency voltage; the improvement which comprises means operatively associated With said intermediate frequency amplifier for adjusting the effective'passband Width of said amplier, means for deriving from the lsaid modulationv volta-ge a uridirec'ti nal control `voltage 'Whose magnitude is substantially directly proportional both rto the modulation voltage ,amplitude and rate of change in modulation voltageamclitude, and means utilizing said control'voltage for regul'ating the effectiveness of `said band width ad-A justing means.

translating channel-*including -af1be1rapass lector, means including an electron discharge tube for adjusting said band pass selector to control the width of the pass band thereof, a frequency-modulation detector coupled to s-aid channel, rectifier means responsive both to the magnitude of the detected signal amplitude and the rate of change in its magnitude for deriving a. control-bias voltage and means for utilizing said control-bias voltage to vary the impedance of said discharge tube in such a manner as to adjust the pass band of said selector directly in accordance with the degree of modulation and rate of change of modulation of the received carrier signal.

8. A frequency-modulatedv carrier-signal receiver comprising, a carrier-frequency signaltranslating channel including fa band-pass selector, means for adjusting said band-pass selector to control the Width of the pass band thereof, a frequency-modulation detector coupled to said channel, rectier means responsive both to the detected signal amplitude4 and rate of change in amplitude for deriving a control-bias voltage, and means for utilizing said controlbias voltage to control said :adjusting means to adjust the pass band of said selector directly in accordance both with the degree of modulation of a received carrier signal and the rate of change in modulation.

EDWIN H. ARMSTRONG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,058,565 Case Oct. 27, 1936 2,083,232 Koch June 8, 1937 2,150,044 Wheeler Mar. 27, 1939 2,152,515 Wheeler Mar. 28, 1939 2,170,202 Kuffmuller Aug. 22, 1939 2,170,475 Hahn Aug. 22, 1939 

